Mexico City is one of the most populated and polluted cities on earth where thick smog usually burns the lungs of its 18 million inhabitants. But a year ago, Mexico City residents, for the first time, elected their own mayor. They chose an opposition politician Cuauhtemoc (cu-wow-TEH-moc) Cardenas (CAR-den-as), who has pledged to make environmental reform a top priority. Bob Carty offers this profile of Mayor Cardenas and the challenges he faces.